docs: crypto: descore-readme.txt: convert to ReST format

Convert this readme file to ReST file format, preserving its
contents as-is as much as possible. The only changes are:

- Added chapter and title markups;
- Added blank lines where needed;
- Added list markups where needed;
- Use a table markup;
- replace markups like `foo' to ``foo``;
- add one extra literal markup to avoid warnings.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/1426be1c7758c0224418352665040220b8a31799.1592203650.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

1 files changed, 414 insertions, 0 deletions

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+.. SPDX-License-Identifier: GPL-2.0
+.. include:: <isonum.txt>
+
+===========================================
+Fast & Portable DES encryption & decryption
+===========================================
+
+.. note::
+
+   Below is the original README file from the descore.shar package,
+   converted to ReST format.
+
+------------------------------------------------------------------------------
+
+des - fast & portable DES encryption & decryption.
+
+Copyright |copy| 1992  Dana L. How
+
+This program is free software; you can redistribute it and/or modify
+it under the terms of the GNU Library General Public License as published by
+the Free Software Foundation; either version 2 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU Library General Public License for more details.
+
+You should have received a copy of the GNU Library General Public License
+along with this program; if not, write to the Free Software
+Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+
+Author's address: how@isl.stanford.edu
+
+.. README,v 1.15 1992/05/20 00:25:32 how E
+
+==>> To compile after untarring/unsharring, just ``make`` <<==
+
+This package was designed with the following goals:
+
+1.	Highest possible encryption/decryption PERFORMANCE.
+2.	PORTABILITY to any byte-addressable host with a 32bit unsigned C type
+3.	Plug-compatible replacement for KERBEROS's low-level routines.
+
+This second release includes a number of performance enhancements for
+register-starved machines.  My discussions with Richard Outerbridge,
+71755.204@compuserve.com, sparked a number of these enhancements.
+
+To more rapidly understand the code in this package, inspect desSmallFips.i
+(created by typing ``make``) BEFORE you tackle desCode.h.  The latter is set
+up in a parameterized fashion so it can easily be modified by speed-daemon
+hackers in pursuit of that last microsecond.  You will find it more
+illuminating to inspect one specific implementation,
+and then move on to the common abstract skeleton with this one in mind.
+
+
+performance comparison to other available des code which i could
+compile on a SPARCStation 1 (cc -O4, gcc -O2):
+
+this code (byte-order independent):
+
+  - 30us per encryption (options: 64k tables, no IP/FP)
+  - 33us per encryption (options: 64k tables, FIPS standard bit ordering)
+  - 45us per encryption (options:  2k tables, no IP/FP)
+  - 48us per encryption (options:  2k tables, FIPS standard bit ordering)
+  - 275us to set a new key (uses 1k of key tables)
+
+	this has the quickest encryption/decryption routines i've seen.
+	since i was interested in fast des filters rather than crypt(3)
+	and password cracking, i haven't really bothered yet to speed up
+	the key setting routine. also, i have no interest in re-implementing
+	all the other junk in the mit kerberos des library, so i've just
+	provided my routines with little stub interfaces so they can be
+	used as drop-in replacements with mit's code or any of the mit-
+	compatible packages below. (note that the first two timings above
+	are highly variable because of cache effects).
+
+kerberos des replacement from australia (version 1.95):
+
+  - 53us per encryption (uses 2k of tables)
+  - 96us to set a new key (uses 2.25k of key tables)
+
+	so despite the author's inclusion of some of the performance
+	improvements i had suggested to him, this package's
+	encryption/decryption is still slower on the sparc and 68000.
+	more specifically, 19-40% slower on the 68020 and 11-35% slower
+	on the sparc,  depending on the compiler;
+	in full gory detail (ALT_ECB is a libdes variant):
+
+	===============	==============	===============	=================
+	compiler   	machine		desCore	libdes	ALT_ECB	slower by
+	===============	==============	===============	=================
+	gcc 2.1 -O2	Sun 3/110	304  uS	369.5uS	461.8uS	 22%
+	cc      -O1	Sun 3/110	336  uS	436.6uS	399.3uS	 19%
+	cc      -O2	Sun 3/110	360  uS	532.4uS	505.1uS	 40%
+	cc      -O4	Sun 3/110	365  uS	532.3uS	505.3uS	 38%
+	gcc 2.1 -O2	Sun 4/50	 48  uS	 53.4uS	 57.5uS	 11%
+	cc      -O2	Sun 4/50	 48  uS	 64.6uS	 64.7uS	 35%
+	cc      -O4	Sun 4/50	 48  uS	 64.7uS	 64.9uS	 35%
+	===============	==============	===============	=================
+
+	(my time measurements are not as accurate as his).
+
+   the comments in my first release of desCore on version 1.92:
+
+   - 68us per encryption (uses 2k of tables)
+   - 96us to set a new key (uses 2.25k of key tables)
+
+	this is a very nice package which implements the most important
+	of the optimizations which i did in my encryption routines.
+	it's a bit weak on common low-level optimizations which is why
+	it's 39%-106% slower.  because he was interested in fast crypt(3) and
+	password-cracking applications,  he also used the same ideas to
+	speed up the key-setting routines with impressive results.
+	(at some point i may do the same in my package).  he also implements
+	the rest of the mit des library.
+
+	(code from eay@psych.psy.uq.oz.au via comp.sources.misc)
+
+fast crypt(3) package from denmark:
+
+	the des routine here is buried inside a loop to do the
+	crypt function and i didn't feel like ripping it out and measuring
+	performance. his code takes 26 sparc instructions to compute one
+	des iteration; above, Quick (64k) takes 21 and Small (2k) takes 37.
+	he claims to use 280k of tables but the iteration calculation seems
+	to use only 128k.  his tables and code are machine independent.
+
+	(code from glad@daimi.aau.dk via alt.sources or comp.sources.misc)
+
+swedish reimplementation of Kerberos des library
+
+  - 108us per encryption (uses 34k worth of tables)
+  - 134us to set a new key (uses 32k of key tables to get this speed!)
+
+	the tables used seem to be machine-independent;
+	he seems to have included a lot of special case code
+	so that, e.g., ``long`` loads can be used instead of 4 ``char`` loads
+	when the machine's architecture allows it.
+
+	(code obtained from chalmers.se:pub/des)
+
+crack 3.3c package from england:
+
+	as in crypt above, the des routine is buried in a loop. it's
+	also very modified for crypt.  his iteration code uses 16k
+	of tables and appears to be slow.
+
+	(code obtained from aem@aber.ac.uk via alt.sources or comp.sources.misc)
+
+``highly optimized`` and tweaked Kerberos/Athena code (byte-order dependent):
+
+  - 165us per encryption (uses 6k worth of tables)
+  - 478us to set a new key (uses <1k of key tables)
+
+	so despite the comments in this code, it was possible to get
+	faster code AND smaller tables, as well as making the tables
+	machine-independent.
+	(code obtained from prep.ai.mit.edu)
+
+UC Berkeley code (depends on machine-endedness):
+  -  226us per encryption
+  - 10848us to set a new key
+
+	table sizes are unclear, but they don't look very small
+	(code obtained from wuarchive.wustl.edu)
+
+
+motivation and history
+======================
+
+a while ago i wanted some des routines and the routines documented on sun's
+man pages either didn't exist or dumped core.  i had heard of kerberos,
+and knew that it used des,  so i figured i'd use its routines.  but once
+i got it and looked at the code,  it really set off a lot of pet peeves -
+it was too convoluted, the code had been written without taking
+advantage of the regular structure of operations such as IP, E, and FP
+(i.e. the author didn't sit down and think before coding),
+it was excessively slow,  the author had attempted to clarify the code
+by adding MORE statements to make the data movement more ``consistent``
+instead of simplifying his implementation and cutting down on all data
+movement (in particular, his use of L1, R1, L2, R2), and it was full of
+idiotic ``tweaks`` for particular machines which failed to deliver significant
+speedups but which did obfuscate everything.  so i took the test data
+from his verification program and rewrote everything else.
+
+a while later i ran across the great crypt(3) package mentioned above.
+the fact that this guy was computing 2 sboxes per table lookup rather
+than one (and using a MUCH larger table in the process) emboldened me to
+do the same - it was a trivial change from which i had been scared away
+by the larger table size.  in his case he didn't realize you don't need to keep
+the working data in TWO forms, one for easy use of half the sboxes in
+indexing, the other for easy use of the other half; instead you can keep
+it in the form for the first half and use a simple rotate to get the other
+half.  this means i have (almost) half the data manipulation and half
+the table size.  in fairness though he might be encoding something particular
+to crypt(3) in his tables - i didn't check.
+
+i'm glad that i implemented it the way i did, because this C version is
+portable (the ifdef's are performance enhancements) and it is faster
+than versions hand-written in assembly for the sparc!
+
+
+porting notes
+=============
+
+one thing i did not want to do was write an enormous mess
+which depended on endedness and other machine quirks,
+and which necessarily produced different code and different lookup tables
+for different machines.  see the kerberos code for an example
+of what i didn't want to do; all their endedness-specific ``optimizations``
+obfuscate the code and in the end were slower than a simpler machine
+independent approach.  however, there are always some portability
+considerations of some kind, and i have included some options
+for varying numbers of register variables.
+perhaps some will still regard the result as a mess!
+
+1) i assume everything is byte addressable, although i don't actually
+   depend on the byte order, and that bytes are 8 bits.
+   i assume word pointers can be freely cast to and from char pointers.
+   note that 99% of C programs make these assumptions.
+   i always use unsigned char's if the high bit could be set.
+2) the typedef ``word`` means a 32 bit unsigned integral type.
+   if ``unsigned long`` is not 32 bits, change the typedef in desCore.h.
+   i assume sizeof(word) == 4 EVERYWHERE.
+
+the (worst-case) cost of my NOT doing endedness-specific optimizations
+in the data loading and storing code surrounding the key iterations
+is less than 12%.  also, there is the added benefit that
+the input and output work areas do not need to be word-aligned.
+
+
+OPTIONAL performance optimizations
+==================================
+
+1) you should define one of ``i386,`` ``vax,`` ``mc68000,`` or ``sparc,``
+   whichever one is closest to the capabilities of your machine.
+   see the start of desCode.h to see exactly what this selection implies.
+   note that if you select the wrong one, the des code will still work;
+   these are just performance tweaks.
+2) for those with functional ``asm`` keywords: you should change the
+   ROR and ROL macros to use machine rotate instructions if you have them.
+   this will save 2 instructions and a temporary per use,
+   or about 32 to 40 instructions per en/decryption.
+
+   note that gcc is smart enough to translate the ROL/R macros into
+   machine rotates!
+
+these optimizations are all rather persnickety, yet with them you should
+be able to get performance equal to assembly-coding, except that:
+
+1) with the lack of a bit rotate operator in C, rotates have to be synthesized
+   from shifts.  so access to ``asm`` will speed things up if your machine
+   has rotates, as explained above in (3) (not necessary if you use gcc).
+2) if your machine has less than 12 32-bit registers i doubt your compiler will
+   generate good code.
+
+   ``i386`` tries to configure the code for a 386 by only declaring 3 registers
+   (it appears that gcc can use ebx, esi and edi to hold register variables).
+   however, if you like assembly coding, the 386 does have 7 32-bit registers,
+   and if you use ALL of them, use ``scaled by 8`` address modes with displacement
+   and other tricks, you can get reasonable routines for DesQuickCore... with
+   about 250 instructions apiece.  For DesSmall... it will help to rearrange
+   des_keymap, i.e., now the sbox # is the high part of the index and
+   the 6 bits of data is the low part; it helps to exchange these.
+
+   since i have no way to conveniently test it i have not provided my
+   shoehorned 386 version.  note that with this release of desCore, gcc is able
+   to put everything in registers(!), and generate about 370 instructions apiece
+   for the DesQuickCore... routines!
+
+coding notes
+============
+
+the en/decryption routines each use 6 necessary register variables,
+with 4 being actively used at once during the inner iterations.
+if you don't have 4 register variables get a new machine.
+up to 8 more registers are used to hold constants in some configurations.
+
+i assume that the use of a constant is more expensive than using a register:
+
+a) additionally, i have tried to put the larger constants in registers.
+   registering priority was by the following:
+
+	- anything more than 12 bits (bad for RISC and CISC)
+	- greater than 127 in value (can't use movq or byte immediate on CISC)
+	- 9-127 (may not be able to use CISC shift immediate or add/sub quick),
+	- 1-8 were never registered, being the cheapest constants.
+
+b) the compiler may be too stupid to realize table and table+256 should
+   be assigned to different constant registers and instead repetitively
+   do the arithmetic, so i assign these to explicit ``m`` register variables
+   when possible and helpful.
+
+i assume that indexing is cheaper or equivalent to auto increment/decrement,
+where the index is 7 bits unsigned or smaller.
+this assumption is reversed for 68k and vax.
+
+i assume that addresses can be cheaply formed from two registers,
+or from a register and a small constant.
+for the 68000, the ``two registers and small offset`` form is used sparingly.
+all index scaling is done explicitly - no hidden shifts by log2(sizeof).
+
+the code is written so that even a dumb compiler
+should never need more than one hidden temporary,
+increasing the chance that everything will fit in the registers.
+KEEP THIS MORE SUBTLE POINT IN MIND IF YOU REWRITE ANYTHING.
+
+(actually, there are some code fragments now which do require two temps,
+but fixing it would either break the structure of the macros or
+require declaring another temporary).
+
+
+special efficient data format
+==============================
+
+bits are manipulated in this arrangement most of the time (S7 S5 S3 S1)::
+
+	003130292827xxxx242322212019xxxx161514131211xxxx080706050403xxxx
+
+(the x bits are still there, i'm just emphasizing where the S boxes are).
+bits are rotated left 4 when computing S6 S4 S2 S0::
+
+	282726252423xxxx201918171615xxxx121110090807xxxx040302010031xxxx
+
+the rightmost two bits are usually cleared so the lower byte can be used
+as an index into an sbox mapping table. the next two x'd bits are set
+to various values to access different parts of the tables.
+
+
+how to use the routines
+
+datatypes:
+	pointer to 8 byte area of type DesData
+	used to hold keys and input/output blocks to des.
+
+	pointer to 128 byte area of type DesKeys
+	used to hold full 768-bit key.
+	must be long-aligned.
+
+DesQuickInit()
+	call this before using any other routine with ``Quick`` in its name.
+	it generates the special 64k table these routines need.
+DesQuickDone()
+	frees this table
+
+DesMethod(m, k)
+	m points to a 128byte block, k points to an 8 byte des key
+	which must have odd parity (or -1 is returned) and which must
+	not be a (semi-)weak key (or -2 is returned).
+	normally DesMethod() returns 0.
+
+	m is filled in from k so that when one of the routines below
+	is called with m, the routine will act like standard des
+	en/decryption with the key k. if you use DesMethod,
+	you supply a standard 56bit key; however, if you fill in
+	m yourself, you will get a 768bit key - but then it won't
+	be standard.  it's 768bits not 1024 because the least significant
+	two bits of each byte are not used.  note that these two bits
+	will be set to magic constants which speed up the encryption/decryption
+	on some machines.  and yes, each byte controls
+	a specific sbox during a specific iteration.
+
+	you really shouldn't use the 768bit format directly;  i should
+	provide a routine that converts 128 6-bit bytes (specified in
+	S-box mapping order or something) into the right format for you.
+	this would entail some byte concatenation and rotation.
+
+Des{Small|Quick}{Fips|Core}{Encrypt|Decrypt}(d, m, s)
+	performs des on the 8 bytes at s into the 8 bytes at
+	``d. (d,s: char *)``.
+
+	uses m as a 768bit key as explained above.
+
+	the Encrypt|Decrypt choice is obvious.
+
+	Fips|Core determines whether a completely standard FIPS initial
+	and final permutation is done; if not, then the data is loaded
+	and stored in a nonstandard bit order (FIPS w/o IP/FP).
+
+	Fips slows down Quick by 10%, Small by 9%.
+
+	Small|Quick determines whether you use the normal routine
+	or the crazy quick one which gobbles up 64k more of memory.
+	Small is 50% slower then Quick, but Quick needs 32 times as much
+	memory.  Quick is included for programs that do nothing but DES,
+	e.g., encryption filters, etc.
+
+
+Getting it to compile on your machine
+=====================================
+
+there are no machine-dependencies in the code (see porting),
+except perhaps the ``now()`` macro in desTest.c.
+ALL generated tables are machine independent.
+you should edit the Makefile with the appropriate optimization flags
+for your compiler (MAX optimization).
+
+
+Speeding up kerberos (and/or its des library)
+=============================================
+
+note that i have included a kerberos-compatible interface in desUtil.c
+through the functions des_key_sched() and des_ecb_encrypt().
+to use these with kerberos or kerberos-compatible code put desCore.a
+ahead of the kerberos-compatible library on your linker's command line.
+you should not need to #include desCore.h;  just include the header
+file provided with the kerberos library.
+
+Other uses
+==========
+
+the macros in desCode.h would be very useful for putting inline des
+functions in more complicated encryption routines.